Debt (Details Textual) (USD $)	3 Months Ended				3 Months Ended	6 Months Ended			6 Months Ended		6 Months Ended				6 Months Ended
	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Feb. 28, 2011 4.60% senior notes due March 2021 [Member]	Mar. 31, 2011 7.65% senior notes [Member]	Jun. 30, 2011 Capital leases and other [Member]	Jun. 30, 2011 Senior notes and debentures [Member]	Dec. 31, 2010 Senior notes and debentures [Member]	Jun. 30, 2011 Tax exempt project bonds [Member]	Dec. 31, 2010 Tax exempt project bonds [Member]	Jun. 30, 2011 Tax exempt bonds [Member]	Dec. 31, 2010 Tax exempt bonds [Member]	Jun. 30, 2011 Canadian credit facility [Member]	Dec. 31, 2010 Canadian credit facility [Member]	Jun. 30, 2011 Revolving Credit Facility [Member]	Jun. 30, 2011 Letter of Credit [Member]
Debt (Textuals) [Abstract]
Weighted average interest rate							6.30%	6.50%	1.30%	2.50%	3.10%	3.10%
Interest rate upper range						0.12	0.0775		0.034		0.074
Interest rate lower range							0.0460		0.001		0.001
Issued of senior notes				$ 400,000,000
Repayment of senior notes matured					147,000,000
Interest rate of Senior notes				4.60%	7.65%
Debt maturing within twelve months, total	321,000,000												144,000,000
Start period of maturity for debt instruments																2013
End period of maturity for debt instruments						2050	2039		2029		2039				2016	2015
Weighted average effective interest rate													2.30%	2.20%
Debt (Additional) (Textuals) [Abstract]
Debt maturing within 12 months classified as long-term	123,000,000
Net Repayment of Canadian credit facility debt	77,000,000
Revolving credit facility, total capacity	2,000,000,000
Canadian credit facility Principal amount outstanding net of discount	144,000,000		212,000,000
Proceeds from the debt issuance		396,000,000
Revolving credit and credit facilities, aggregate capacity	2,500,000,000
Letter of credit outstanding, revolving credit and letter of credit facilities	1,500,000,000
Letter of credit outstanding, revolving credit facility	$ 1,100,000,000